Heat sensitive resistor



Dec. 13, 1955 R. L. LONGlNl ET AL 2,727,113

HEAT SENSITIVE RESISTOR Filed Dec. 29, 1951 Fig.2

A i a T i C F 59.3. VAT 7 v5 l l WITNESSES: INVENTORS Richard L.Longini and Donald K.Coles.

' ATTORNEY United States PatentO HEAT SENSITIVE RESISTOR Richard L. Longini and Donald K. :Coles, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation .of Penusylrania Application December 29, 1951, Serial No. 264,196

9 Claims. ((31.201-63) Our invention relates to devices employing exceedingly thin films and in particular relates to films which are capable of changing temperature rapidly due to vary ing energy infiux. To give a specific example, a rapidly varying influx of infra-red radiation may be measured and recorded by using an electric discharge tube with a thermionically emissive cathode which is heated by absorbing the incident infra-red radiation.

While many materials are known which are thermionically responsive in high degree, a moments consideration will show that to satisfactorily respond to a rapidly varying energy influx the temperature change must have an extremely small time-constant. In structures having the dimensions which are ordinarily met with in everyday life substantial temperature change is a relatively slow procedure. It is clear therefore that a thermionically responsive element that will respond copiously to rapid changes of energy influx must have rather unusual properties. Since the time constant of temperature change in a given material is directly proportional to its mass, it is possible to attain a desirably rapid response to changes in energy influx by making the mass of the energy absorbing material small enough; that is to say, by sufliciently reducing the dimension of the absorber in a direction parallel to that of energy inflow. However, it is evi dent that any supporting material which contacts the thermionic material must follow its temperature changes; that is to say, the combined thickness of the thermionically responsive material and any supporting material for it must be exceedingly thin if a rapid following of inflowing energy is to be achieved.

One object of our invention is accordingly to provide methods of supporting films of thermionic materials which shall have small time constants of response to energy influx.

Another object of our invention is to provide methods of forming and supporting films of thermionically responsive material which are exceedingly thin.

Another object of our invention is to provide a novel method of forming exceedingly thin films of material supported at points which are relatively wide apart.

Another object of our invention is to produce a novel form of electrical detector for incident radiation.

Another object of our invention is to produce a novel thermionic device responding to radiation incident thereon.

Still another object is to produce a novel thermionic device responding to incident infra-red radiation.

Another object is to produce a novel thermionic device responding to incident ultra-violet radiation.

Yet another object is to produce an improved thermionically emissive cathode.

A further object is to produce a novel form of thermoelectric junction.

Yet another object is to produce a novel type of bolometer element.

Other objects of our invention will be apparent to those skilled in the art upon reading the following description taken in connection withthe drawings, in which:

Figure 1 is a detail view in section to enlarged scale showing a film and supports therefor made in accordance with my invention;

Fig. 2 is a plan view,

Fig. 3 an elevational section of a thermocouple formed in accordance with our invention; and

Fig. 4 is a plan view of a bolometer made in accordance with our invention.

Referring in detail to Fig. 1, a base plate 1 of any material suitable for the use to which the thin film of our invention is to be used has its upper surface provided with fine projecting points 2 of small tip-diameter spaced apart by distances large relative to the thickness of the film to be formed. This elevation of the tip of the points above the plate will usually also be considerably larger than the thickness of the film, though this is not desirable for all cases. The spacing between the points may be variable, even random, but its mean value should follow the above prescription. One way in which we have found it possible to produce such points is to abrade the surface of a polished plate with coarse emery. Another more satisfactory, for many purposes, is to scatter fine particles of some crystal over the polished plate. Crystals ordinarily have apieces of small tip-diameter, each apex rising to a point. To take a specific example, a thin film detector of infra-red radiation may comprise a polished plate of silver chloride which has crystals averaging one micron in diameter of the same material scattered over it with an average spacing of about .003 cm. by settling through water. A few drops of water are then placed upon the plate and a thin plastic film of organic material is formed on the water; for example, by the spreading of a drop of collodion solution or by spraying that solution.

The plastic-covered plate is then slowly dried and put in a vacuum system where a thin layer 3 of the material for the thin film desired is sublimed onto the collodion. For the specific plate described above, a silicon monoxide layer is thus formed. The sublimation is stopped when the desired thickness for the final film is reached. For the silicon monoxide this thickness is about a thousand Angstrorns. The unit is now removed from the vacuum system and heated in air or oxygen at a temperature suflicient to burn out the plastic, Whereupon the supernatant thin layer is supported like a tent from point to point of the crystal particles. The important feature is that the film hang from the pointed projections without touching the back-plate. By reason of its extreme thinness the incidence, on any free area of the film, of a small amount of inflowing energy, e. g. infra-red radiation, heats that area very rapidly. To cause emission of electrons from any such area the silicon monoxide film is coated by evaporation with a thin layer of silver, the latter is oxidized by surrounding it with oxygen and producing a glow discharge therein, and then replacing the oxygen atmosphere with caesium vapor.

Where the thin sheet of silicon monoxide with thermionic emitter thereon formed as above-described is used as cathode for an electrical discharge tube, the anode current thus follows with only a very small time-lag the fluctuations with time of radiation incident on the silicon monoxide film and can be used to register on an oscilloscope a curve measuring the fluctuations with time of the radiation flux. It is also possible by ways well known in the electronics art to use the same current to control numerous other types of apparatus in response to the variations of radiation inflow. The thin emitting film described here may be used to replace a photosurface in any of the various types of television camera tubes thus resulting in a long wave length infrared picture tube.

It is possible to employ our invention to generate currents thermoelectrically in response to energy influx by use of a structure shown schematically in Figs. 2 and 3. To produce this arrangement, the silicon monoxide film formed as above-described has a mask applied to its surface extending from one end to a point a little short of the mid-point, as at A in Fig. 2, before it is placed in the vacuum chamber. It is then inserted in the chamber and coated by evaporation with one of the metals desired to constitute the thermocouple. Once a film just thick enough to be handled is formed, the mask is removed, and the coated portion is masked for a similar distance from the other end, as at B in Fig. 2. On replacement in the vacuum chamber, the unit is coated by evaporation with the other metal desired for the thermocouple. In the interval between A and B in Fig. 2, there is thus formed a small area in which both metals are in contact with each other; and so there is produced a thermoelectric junction of very small mass per unit of width of the base-film. The combined unit may then be provided by thickened terminals 4, 5, by evaporation, electrode-position or the like, if desired, before burning out the plastic baselayer. The width of the unit in direction C may of course be as small as is consistent with ease in handling. The terminals 4, 5 may be of any material convenient for connection with an electric circuit, but the lengths A and B should be great enough so that the terminals remain at the cold junction temperature despite the heating effect of the radiation incident on the thermal junction between A and B.

It may be desirable for some purposes to form a thermocouple by depositing on the plastic base a thin layer of non-conductive refractory material, before forming the two layers of metal which form the thermoelectric junction. This may be done by evaporating in the vacuum chamber a layer of silicon monoxide before applying the first mask in the procedure just described. After that is done, the steps outlined above for masking and depositing the metal layers may be followed.

The principles of our invention may be used to produce a bolometer of minute mass as shown schematically in Fig. 4.

A thin film of non-conductive material is formed as described above in connection with Fig. 2 and is then masked to leave a narrow zig-zag area like 6 in Fig. 4. As will be seen, this free area is a continuous narrow line starting at the upper left corner and ending at the lower right corner of the plate. The unit is then placed in the vacuum chamber and coated by evaporation with a thin layer of the desired metal or semi-conductor which is to form the bolometer element. After this, it is taken from the chamber, the mask removed, thickened terminals provided, and the plastic burned out in accordance with the description of Figs. 1 to 3.

While we have referred to silicon monoxide as a specific instance of a material to be formed into a thin film many other materials may be used as will be evident to those skilled in the art. Zirconium may be mentioned as one example.

We claim as our invention:

1. An electrothermal response element comprising a base thick enough for mechanical strength supporting a layer having a large percentage of vacant space, a

thin film of silicon monoxide supported by the face of said layer, a thermionically emissive surface on said film, and a current lead connected to said film.

2. An electrothermal response element comprising a base thick enough for mechanical strength supporting widely separated crystals having small tip diameters, a thin film of electrothermal response material supported by apices of said crystals, and a current lead connected to said film.

3. An electrothermal response element comprising a base of silver chloride, crystals of silver chloride separated from each other by wide spaces on the surface of said base plate, a thin film of silicon monoxide supported on apices of said crystals, a thin film of caesiumsilver oxide formed on the silicon monoxide, and a current lead connected to one of said films.

4. An electrothermal response element comprising a base of silver chloride, crystals of silver chloride separated from each other by wide spaces on the surface of said base plate, and a thin film of silicon monoxide supported on apices of said crystals, a thermionically emissive surface on said film, and a current lead connected to said film.

5. An electrothermal response element comprising a base thick enough for mechanical strength supporting widely separated projections of small tip diameter, a thin film of insulating material supported on said tips, a thin film of material having a substantial temperature coefficient of resistance supported on said insulating material, and a current lead connected to the last-said film.

6. An electrothermal response element comprising a base thick enough for mechanical strength supporting widely separated projections of small tip diameter, a thin film having a thermionically emissive surface supported on said tips, and a current lead connected to said film.

7. An electrothermal response element comprising a base thick enough for mechanical strength supporting widely separated projections of small tip diameter, a first thin film of one metal supported by said tips over part of the surface of said base, a second thin film of a different metal supported over another part of said surface, said films contacting each other in a junction of only small area, and current leads connected respectively to said first and second films.

8. An electrothermal response element comprising a base thick enough for mechanical strength supporting widely separated projections having small tip diameters, a thin film of electrothermal response material supported by the tips of said projections, and a current lead connected to said film.

9. An electrothermal response element comprising a base thick enough for mechanical strength supporting widely separated projections of small tip diameter, a thin film having an electron emissive surface supported on said tips, and a current lead connected to said film.

References Cited in the file of this patent UNITED STATES PATENTS 1,807,326 Ruben May 26, 1931 2,289,921 Masso July 14, 1942 2,374,310 Shaefer Apr. 24, 1945 2,393,196 Schwarz Jan. 15, 1946 2,489,127 Forgue Nov. 22, 1949 2,597,617 Campbell May 20, 1952 

